Research and development of a planar lightwave circuit (PLC) composed of a silica-based glass waveguide formed on a silicon substrate have been actively conducted. An arrayed waveguide grating (AWG) utilizing such PLC technique is a circuit for optical wavelength multiplexing and demultiplexing and plays an important role as a component for optical communications.
The AWG has temperature dependence on a transmission wavelength of light to be multiplexed and demultiplexed. This is because the effective refractive index of a silica-based glass waveguides constituting the AWG has temperature dependence. Therefore, normally in an AWG, a temperature adjusting device is required in order to keep the wavelength transmission characteristics constant.
In order to eliminate the temperature adjusting device additionally required for an AWG, a method of reducing the temperature dependence of transmission wavelength of an AWG has been developed. This method is disclosed in, for example, Patent Literatures 1 and 2 (PTL 1 and PTL 2). The AWG with the temperature dependence of transmission wavelength being reduced is referred to as a temperature insensitive AWG or an athermal AWG. The athermal AWG disclosed in Patent Literatures 1 and 2 is realized by forming a groove so as to intersect a light wave traveling axis and inserting into the groove a material having a temperature coefficient of refractive index different from the temperature coefficient of effective refractive index of the waveguide (hereinafter referred to as a “temperature compensating material”) in each optical path (arrayed waveguide or slab waveguide) within the AWG.
FIG. 35A is a diagram showing a conventional configuration example of an athermal AWG of a type forming a groove in a slab waveguide. The athermal AWG 4100 comprises first input/output waveguides 4101, a first slab waveguide 4102, arrayed waveguides 4103, a second slab waveguide 4104, second input/output waveguides 4105 and grooves 4106. The grooves 4106 are filled with a temperature compensating material. In this configuration example, the grooves 4106 are formed in the first slab waveguide 4102.
FIG. 35B is a diagram showing a sectional structure along a segment XXXVB-XXXVB′ in FIG. 35A. As shown, on a silicon substrate 4107, a waveguide core 4108 and a clad 4109 of the slab waveguide 4102 are formed. The groove 4106 is formed by removing a part of the waveguide core 4108 and the clad 4109 so as to divide the waveguide core 4108. In FIG. 35A and FIG. 35B, the grooves 4106 are divided into a plurality of grooves. This is because radiation loss can be reduced compared to a single groove.
In the athermal AWG 4100, wavelength multiplexed signal light input to the first input/output waveguide 4101 is demultiplexed to each waveguide of the second input/output waveguides 4105 and output as signal light for each wavelength channel. Further, the signal light for each wavelength channel input to each waveguide of the second input/output waveguides 4105 is multiplexed to the first input/output waveguide 4101 and output as wavelength multiplexed signal light. That is, the athermal AWG operates as an optical wavelength multiplexing and demultiplexing circuit.
In FIG. 35A, a length Li of an ith waveguide of the arrayed waveguides 4103 is expressed as Li=L1+(i−1)·ΔL and the arrayed waveguides 4103 are designed such that the waveguide length increases successively by a fixed amount ΔL. In parallel with this, a total length Li′ for the light wave incident to each arrayed waveguide to be disturbed by the grooves 4106 in the first slab waveguide 4102 is expressed as Li′=L1′+(i−1)·ΔL′ and the grooves are shaped so as to increase successively by an amount ΔL′ in proportion to ΔL. In this case, a transmission center wavelength λc from the first input/output waveguide 4101 to the center waveguide of the second input/output waveguides 4105 of the AWG is expressed by the following:λc={naΔL−nsΔL′+n′ΔL′}/M.   (1)where, na is the effective refractive index of the arrayed waveguides, ns is the effective refractive index of the slab waveguides, n′ is the refractive index of the temperature compensating material, and M is the diffraction order. Further, {naΔL−nsΔL′+n′ΔL′} indicates a difference in distance between neighboring optical paths in the AWG, i.e., an optical path length difference. In this case, it is assumed that n′ is close to ns and the refraction angle of a light wave in the grooves is sufficiently small. Here, the optical path length is a distance over which a light wave experiences and is found by a product of a refractive index of a material and a physical path distance.
The athermal AWG is designed to hold ΔL′/(ΔL−ΔL′)=−α/α′, i.e., ΔL′=ΔL/(1−α′/α) where the temperature coefficient of effective refractive index of the arrayed waveguides and the slab waveguides is α (α=dna/dT=dns/dT, T is temperature) and the temperature coefficient of refractive index of the temperature compensating material is α′ (α′=dn′/dT). Due to this, the change with temperature in optical path length difference in the arrayed waveguides and the slab waveguides is canceled by the change with temperature in the optical path length difference of the temperature compensating material filled in the groove, and therefore, the temperature dependence at the transmission center wavelength is compensated. As the temperature compensating material, any material can be used as long as it has α′ that satisfies the above-described conditions for the α of the waveguide. However, a material is preferable, for which the sign of α′ is different from that of α and |α′| is sufficiently greater than |α|. This is because the ΔL′ can be designed to be small and the excess loss due to the grooves can be suppressed. A material of such conditions is, for example, silicone resin, which is optical resin, and α′ is about −35×α. Further, the optical resin is also preferable since it is superior in long-term reliability as an optical component material.
As another method of reducing the temperature dependence of transmission wavelength of the AWG, a method is known, in which the chip of the AWG is cut into an arc-shape along the circuit, both ends of the chip are joined by a metal rod, so that the AWG chip is deformed by thermal expansion and contraction of the metal rod, thereby cancelling the change with temperature in the optical path length difference of the neighboring arrayed waveguides. The details are disclosed in Non-Patent Literature 1 (NPL 1).
As still another method of reducing the temperature dependence of transmission wavelength of the AWG, a method is known, in which the slab waveguide on the input side or output side of the AWG chip is divided, the divided chips are joined by a metal plate, so that the relative position of the divided slab waveguides is changed by thermal expansion and contraction of the metal plate. This can cancel the change with temperature in the optical path length difference of the arrayed waveguides.
In the conventional AWG, a transmission spectrum is an integration of power overlap of the photoelectric field excited at the connection boundary between the first input/output waveguide and the first slab waveguide and the photoelectric field excited at the connection boundary between the second input/output waveguides and the second slab waveguide. Normally, in these photoelectric fields, only the fundamental mode is excited and the transmission spectrum waveform has a shape of the Gaussian function. However, a method has been developed for flattening a transmission waveform to extend the band by providing a parabolic tapered waveguide at the connection part of the first input/output waveguide to the first slab waveguide or the connection part of the second input/output waveguides to the second slab waveguide 4104. The details are disclosed in Patent Literature 3 (PTL 3).